Various processes are known for the conversion of hydrocarbonaceous feedstocks, especially methane from natural sources, for example natural gas, associated gas and/or coal bed methane, into liquid products, especially methanol and liquid hydrocarbons, particularly paraffinic hydrocarbons. At ambient temperature and pressure these hydrocarbons may be gaseous, liquid and (often) solid. Such processes are often required to be carried out in remote and/or offshore locations, where no direct use of the gas is possible. Transportation of gas, for example through a pipeline or in the form of liquefied natural gas, requires extremely high capital expenditure or is simply not practical. This holds true even more in the case of relatively small gas production rates and/or fields. Re-injection of gas will add to the costs of oil production, and may, in the case of associated gas, result in undesired effects on crude oil production. Burning of associated gas has become an undesirable option in view of depletion of hydrocarbon sources and air pollution. A process often used for the conversion of carbonaceous feedstocks into liquid and/or solid hydrocarbons is the well-known Fischer-Tropsch process.
For a general overview for the Fischer-Tropsch process reference is made to Fischer-Tropsch Technology, Studies in Surface Science and Catalysis, Vol. 152, Steynberg and Dry (ed.) Elsevier, 2004, Amsterdam, 0-444-51354-X. Reference is further made to review articles in Kirk Othmer, Encyclopedia of Chem. Techn. and Ullmann's Encyclopedia of Ind. Chem.
The Fischer-Tropsch process can be used for the conversion of hydrocarbonaceous feed stocks into especially liquid and/or solid hydrocarbons. The feed stock (for example natural gas, associated gas, coal-bed methane, residual (crude) oil fractions, peat, biomass or coal) is converted in a first step into a mixture of hydrogen and carbon monoxide (this mixture is referred to as synthesis gas or syngas). The syngas is then converted in one or more steps over a suitable catalyst at elevated temperature and pressure into mainly paraffinic compounds ranging from methane to high molecular weight molecules comprising up to 200 carbon atoms, or, under particular circumstances, even more.
Depending on the catalyst and the process conditions used in the Fischer-Tropsch reaction, normally gaseous hydrocarbons, normally liquid hydrocarbons and optionally normally solid hydrocarbons are obtained. It is often preferred to obtain a large fraction of normally solid hydrocarbons. In that case a high selectivity to C5+-hydrocarbon is obtained. These solid hydrocarbons may be obtained up to 85 wt % based on total hydrocarbons, usually between 50 and 75 wt %. The term “normally” relates to STP-condition (i.e. 0° C., 1 bar).
The partial oxidation of gaseous feedstocks, producing mixtures of especially carbon monoxide and hydrogen, can take place according to various established processes. These processes include the Shell Gasification Process. A comprehensive survey of this process can be found in the Oil and Gas Journal, Sep. 6, 1971, pp 86-90.
Generally, the partial oxidation process looks to convert natural gas, which is mainly methane, to the carbon monoxide and hydrogen mixture known as syngas. Pure methane would create a theoretical hydrogen to carbon monoxide (hereinafter termed “H2/CO”) molar ratio of 2, but because natural gas includes other compounds such as ethane, and because sometimes excess oxygen is used to try and achieve substantial, close or near 100% conversion of the methane, the actual H2/CO ratio in syngas is usually less than 2, such as 1.7-1.8. In the case of residual oil fractions, peat, (brown) coal and biomass the H2/CO ratio is usually between 1.7 and 0.6.
The Fischer-Tropsch (FT) process may be operated in a single pass mode (“once through”) or in a recycle mode. In either configuration, there is usually only one syngas entry stream into the process reactor or reactors. Meanwhile, it is desired to obtain an overall CO conversion level or percentage as high as possible.